1. Field of the Invention
This invention relates to computer aided manufacturing and more specifically to a method and apparatus for generating a computer numerical control program for controlling a numerical control machine.
2. Background Information
In milling a workpiece using a numerical control (NC) machine, it is desirable to remove material from the workpiece as fast as possible consistent with long tool life.
Methods for achieving a fast material removal rate simultaneously with long tool life are described in U.S. Pat. No. 7,451,013, the contents of which are incorporated herein by reference in their entirety. The methods described in U.S. Pat. No. 7,451,013 remove material in multiple intermediate phases, each phase employing a particular type of tool path which is most suitable for the shape of the material to be removed. As a consequence of the aforementioned material removal methods, each intermediate phase leaves one or more regions of the workpiece to be removed in a subsequent intermediate phase until the final finish phase is completed.
FIG. 1 is an illustration of a general case where material to be removed from a workpiece has been left in an interior region of the workpiece at the completion of an intermediate phase of milling. The region can be characterized as having three sides, a first side which can be a part boundary, i.e., a workpiece boundary existing at the beginning of the milling operation or a boundary to be attained at the completion of the milling operation, or an in-process material boundary, i.e., a boundary of the workpiece established by an intermediate milling operation, and second and third sides which may be either part boundaries or in-process material boundaries. An inside corner of a pocket is typical of the type of region left to be milled from a previous intermediate phase. Such a region would be characterized, for example, as having an in-process material boundary for a first side, and part boundaries for the second and third sides.
FIG. 2 illustrates a tool path for milling the region of FIG. 1 by one of the methods described in U.S. Pat. No. 7,451,013. In this method, the tool path for milling the region consists of a series of circularly shaped tool passes that successively advance into the region from the first side, removing material with tool passes that traverse between the third side and the second side; and transition passes, which do not remove material, that return the milling cutter from the second side to the third side after each material removing pass in preparation for the next material removing tool pass.
While such a tool path is desirable from the point of view of tool life, a drawback to the foregoing method is that the length of each transition pass required to return the tool from each material removing tool pass to a location from which the following removal tool pass can begin is approximately equal to the length of the tool pass which cuts metal. Because such transition passes do not remove material, the material removal efficiency of this method is limited in that only about half of the total length of the tool path is used to remove material.
In consideration of the above, it would be desirable to have a tool path for milling a region of a workpiece which has shorter transition, i.e., non-material removing passes.